1. Field of the Invention
The present invention relates to a light articulation ball-joint and to a method of manufacture of such a ball-joint.
2. Brief Description of the Related Art
In the aeronautics industry it is known to use a metal ball-joint for immobilising two parts, such as a connecting rod and a structural part, in translation relative to each other, while permitting a relative rotational movement of the parts. Such a ball-joint, described for example in FR-A-2 770 597, conventionally comprises a metal inner ring and a metal outer ring. An inner surface of the inner ring is adapted to be mounted on a shaft, while its outer surface is substantially spherical and suitable for co-operating with a corresponding inner surface of the outer ring. The inner ring and the outer ring thus have three mutual degrees of freedom in rotation, while being firmly fixed to each other in translation.
In order to obtain a light metal ball-joint, it is advantageous to form the inner and outer rings of such a ball-joint from a metal or from a metal alloy having a low density. In particular, titanium, titanium alloys, aluminium and aluminium alloys make it possible to produce light rings having a high mechanical strength. However, the frictional behaviour of the aforesaid metals and metal alloys is not compatible with the stresses applied, in operation, to the rings of an articulation ball-joint. The relative movements of the rings when under load are accompanied by friction, which generates wear phenomena due to fretting and to sliding at the contact surfaces of the rings. The degradation of these contact surfaces results in loosening of the ball-joint and an increase in the coefficient of friction at the interface between the rings. In particular, there is a risk that peaks of roughness, present on the rubbing surfaces owing to the effect of wear by fretting, weld to each other, which may, under a high load, lead to complete locking or jamming of the ball-joint.
Light ball-joints comprising inner and outer rings made of titanium alloy are moreover known in which the spherical surfaces are provided with specific coatings, such as very hard thin films, applied to the surfaces of the rings by Physical Vapour Deposition (PVD). These coatings make it possible to improve the frictional properties at the interface between the rings. However, owing to the fact that they are very thin, the coatings have a tendency to be eliminated rapidly during the relative movements of the rings under a high load, and therefore have a limited operating range in terms of contact pressure. When the coating of one of the rings is completely eliminated, the titanium alloy surface of the ring comes into contact with the coating of the other ring. The result of this is rapid degradation of the titanium alloy surface, capable of leading to jamming of the ball-joint, thereby severely limiting the service life of the ball-joint.
It is these drawbacks which the invention is more particularly intended to remedy by proposing a light articulation ball-joint having a stable behaviour in operation and an improved service life.